The present invention relates to a light diffusion sheet, in particular, a light diffusion sheet suitable for use in backlight units of liquid crystal displays.
As light diffusion sheets used for backlight units of liquid crystal displays, there are conventionally used transparent plastic films applied on one surface with a transparent resin solution containing inorganic particles or resin particles dispersed therein.
Performance features required by such light diffusion sheets include invisibility of light diffusion patterns in the light conductive plates, high luminance in the front direction and so forth.
To realize these features, improvements have been made by selecting the type and content of the resin and light diffusion particles used in the light diffusion layers. However, the enhancement of luminance in the front direction that can be achieved by such improvements is limited. Therefore, attempts have been made to direct light in the peripheral direction toward the front direction by means of a prism sheet. Since such a prism sheet does not have light-diffusing ability, the practice has been to superimpose it on a conventional light diffusion sheet. Japanese Patent Unexamined Publication (Kokai) Nos. 9-127314 and 9-197109, for example, disclose light diffusion sheets which, being combined with a lens sheet called a prism sheet, provides enhanced luminance in the front direction compared with conventional light diffusion sheets, together with adequate light-diffusing property.
Such light diffusion sheets as disclosed in Japanese Patent Unexamined Publication (Kokai) Nos. 9-127314 and 9-197109 aim at obtaining high luminance and excellent light diffusion by using a prism sheet. However, such a prism sheet has drawbacks. For example, it is expensive and it is susceptible to surface damage, making it difficult to handle. From the viewpoint of cost performance, therefore, a strong need has recently come to be felt for a light diffusion sheet that can realize high luminance and excellent light diffusion without using such a prism sheet.
Accordingly, an object of the present invention is to provide a light diffusion sheet that has high luminance in the front direction, is excellent in light-diffusing property, does not require use of an expensive prism sheet which is readily susceptible to surface damage, and does not require careful handling.
In the light of the above-mentioned object, the inventors conducted assiduous studies on improvement of contradictory properties, i.e., excellent light diffusion and high luminance in the front direction. As a result, they found that, when the light diffusion layer has high haze and a high distinctness of transmission image, the light diffusion layer exhibits excellent light diffusion property and yet high luminance in the front direction, and accomplished the present invention.
That is, the present invention provides a light diffusion sheet comprising a light diffusion layer formed on a transparent substrate and containing a binder resin and resin particles that impart an uneven surface, wherein the light diffusion sheet has a total light transmission of 70.0% or more, haze of 80% or more and a distinctness of transmission image of 25.0% or more.
In the light diffusion sheet of the present invention, internal haze of the light diffusion sheet is preferably less than 40.0%. The internal haze is haze caused by difference between refractive indices of the binder resin and the resin particles in the light diffusion layer, being distinguished from haze caused by the uneven surface of the light diffusion sheet.
Further, in the light diffusion sheet of the present invention, the difference between refractive indices of the binder resin and the resin particles is preferably 0.05 or less.
In this specification, the total light transmission and haze are those defined in JIS-K7105, and they satisfy the following relationship: Td(%)/Tt(%)xc3x97100(%)=H(%)[Td: diffused light transmission, Tt: total light transmission, H: haze].
In this specification, the distinctness of transmission image is that defined in JIS-K7105, i.e., a value obtained by the following equation using a maximum wave height (M) and a minimum wave height (m) measured by the transmission method with an optical comb having a width of 2.0 mm.
A distinctness of transmission image [C(2.0)]={Mxe2x88x92m}/{M+m}xc3x97100(%)
In this specification, the refractive index is that of D-line measured at 25xc2x0 C. (n25D).
According to the present invention, since the light diffusion sheet, which comprises a light diffusion layer formed on a transparent substrate and containing a binder resin and resin particles that impart an uneven surface, has the above-mentioned optical properties (a total light transmission, haze, a distinctness of transmission image), it has high luminance in the front direction, is excellent in light-diffusing property and cost performance, and does not require use of an expensive prism sheet which is readily susceptible to surface damage.